1. Field of the Invention
This invention relates generally to the field of geophysical prospecting and particularly to the field of marine seismic surveys. More particularly, the invention relates to noise attenuation in marine seismic streamers.
2. Description of the Related Art
In the oil and gas industry, geophysical prospecting is commonly used to aid in the search for and evaluation of subterranean formations. Geophysical prospecting techniques yield knowledge of the subsurface structure of the earth, which is useful for finding and extracting valuable mineral resources, particularly hydrocarbon deposits such as oil and natural gas. A well-known technique of geophysical prospecting is a seismic survey. In a land-based seismic survey, a seismic signal is generated on or near the earth's surface and then travels downwardly into the subsurface of the earth. In a marine seismic survey, the seismic signal may also travel downwardly through a body of water overlying the subsurface of the earth. Seismic energy sources are used to generate the seismic signal which, after propagating into the earth, is at least partially reflected by subsurface seismic reflectors. Such seismic reflectors typically are interfaces between subterranean formations having different elastic properties, specifically wave velocity and rock density, which lead to differences in acoustic impedance at the interfaces. The reflections are detected by seismic sensors at or near the surface of the earth, in an overlying body of water, or at known depths in boreholes. The resulting seismic data is recorded and processed to yield information relating to the geologic structure and properties of the subterranean formations and their potential hydrocarbon content.
Appropriate energy sources may include explosives or vibrators on land and air guns or marine vibrators in water. Appropriate types of seismic sensors may include particle velocity sensors in land surveys and water pressure (typically pressure gradient) sensors in marine surveys. Particle acceleration sensors may be used instead of particle velocity sensors. Particle velocity sensors are commonly known in the art as geophones and water pressure sensors are commonly know in the art as hydrophones. Both seismic sources and seismic sensors may be deployed by themselves or, more commonly, in arrays.
In a typical marine seismic survey, a seismic survey vessel travels on the water surface, typically at about 5 knots, and contains seismic acquisition equipment, such as navigation control, seismic source control, seismic sensor control, and recording equipment. Seismic streamers cables are elongate cable-like structures towed in the body of water by the seismic survey vessel that tows the seismic source or by another seismic survey ship. Typically, a plurality of seismic streamers is towed behind a seismic vessel. The seismic streamers contain sensors to detect the wavefields reflected from reflecting interfaces. Conventionally, the seismic streamers contain pressure sensors such as hydrophones, but seismic streamers may also contain particle motion sensors such as geophones or accelerometers. The pressure sensors and particle velocity sensors may be deployed in close proximity, collocated in pairs or pairs of arrays along a seismic cable.
The sources and streamers are submerged in the water, with the seismic sources typically at a depth of 5-15 meters below the water surface and the seismic streamers typically at a depth of 5-40 meters.
The recorded seismic data signal contains useful primary reflections as well as much noise. The recorded noise may be coherent (that is, acts like a traveling wave) or random. Examples of coherent noise in land surveys include ground roll, guided waves, side-scattered noise, cable noise, air wave, power lines, and multiples. Multiples are especially strong relative to primaries in marine seismic surveys, because the water-earth and, particularly, the air-water interfaces are strong seismic reflectors due to their high acoustic impedance contrasts. Examples of random noise include noise resulting from poorly planted geophones and wind motion in land surveys, and noise resulting from transient movements in the seismic streamer cable, wave motion in the water causing the cable to vibrate, electrical noise from the recording instruments, and scattered noise from the many reflection surfaces in the subsurface in marine surveys.
Marine seismic streamers are typically divided into streamer sections approximately 100 meters in length, and can extend to a length of thousands of meters. A typical streamer section includes an external jacket, strength members, spacers, an electrical wire bundle, and connectors. The external jacket protects the interior of the streamer section from water ingress. The strength members, usually two or more, run down the length of each streamer section from end connector to connector, providing axial mechanical strength. The spacers maintain the cylindrical shape of the streamer section. The electrical wire bundle also runs down the length of each streamer section, and includes electrical power conductors and electrical and/or optical fiber communication conductors. Connectors at the ends of each streamer section link the section mechanically, electrically or optically to adjacent sections and, hence, ultimately to the seismic towing vessel.
Sensors, typically hydrophones or arrays of hydrophones, are located within the streamer. The hydrophones have sometimes been located within the spacers for protection. The distance between spacers is normally about 0.7 meters. An array of sensors, typically comprising 8 or 16 hydrophones, normally extends for a length of about 6.25 meters or 12.5 meters, respectively. These array lengths allow 16 or 8 arrays, respectively, in a standard 100 meter seismic section.
The interior of the seismic streamers is typically filled with a core material to provide buoyancy and desirable acoustic properties. For many years, most seismic streamers have been filled with a fluid core material. A drawback to using fluid-filled streamer sections is the noise generated by vibrations as the streamer is towed through the water. These vibrations develop internal pressure waves traveling through the fluid in the streamer sections, which are often referred to as “bulge waves” or “breathing waves”.
In addition, there are other types of noise, often called flow noise, which can affect the hydrophone signal. For example, vibrations of the seismic streamer can cause extensional waves in the external jacket and resonance transients traveling down the strength members. A turbulent boundary layer created around the outer skin of the streamer by the act of towing the streamer can also cause pressure fluctuations in the fluid core material. The extensional waves, resonance transients, and turbulence-induced noise are typically much smaller in amplitude than the bulge waves. Even thermal variations within the streamer can bring about thermal stress in hydrophones employing piezoelectric materials, causing noise. Bulge waves are usually the largest source of vibration noise because these waves travel in the fluid core material filling the streamer sections and thus act directly on the hydrophones.
Several approaches have been employed to reduce the bulge noise problem in fluid filled steamer sections. For example, one approach is the use of stretch sections at the front and rear of the seismic streamer. Another approach is the application of low-cut filters. Another approach is to introduce compartment blocks in the sections to impede the vibration-caused bulge waves from traveling continuously along the streamer. Another approach is to introduce open cell foam into the interior cavity of the streamer section. The open cell foam restricts the flow of the fluid fill material in response to the transient pressure change and causes the energy to be dissipated into the external jacket and the foam over a shorter distance.
Another approach for eliminating bulge noise is to eliminate the fluid from the streamer sections, so that no medium exists in which bulge waves can develop. This approach is exemplified by the use of streamer sections filled with a solid core material or softer solid material instead of a fluid. However, in any solid type of material, some shear waves will develop, which can increase the noise detected by the hydrophones.
Another approach to address the noise problem is to combine several hydrophones into an array (also known as a group) to attenuate a slow moving wave. Traditional array forming for noise suppression in marine seismic streamers is based on a number of sensors (normally 8 to 16) connected together and effectively summed within the streamer sections before analogue to digital conversion.
However, there is a continuing need for methods for more effectively attenuating noise in marine seismic streamers.